The present invention relates to the field of signal amplification, and more particularly, to a method and apparatus for amplifying a radio frequency (RF) signal employed, for example, in wireless communication devices.
Typical RF communication devices (or similar wireless devices such as cellular phones, for example) receive RF signals from a RF transmission source or similar transmission base (such as a cellular phone transmission tower or RF base unit, for example). If the operator is located near the RF transmission source, the RF communication device does not need to amplify or provide gain to the RF signal received because the RF signal received by the communication device is strong enough to be easily processed by the communication processing circuits within the RF communication device. However, if the operator is further away from the RF base as determined by the signal strength from signal strength indicator circuitry (such as, for example, item 61 in FIG. 1, which could include a received signal strength indicator or RSSI), the RF signal received by the RF communication device may be weak. Further, the RF signal may be near other signals having the same or similar frequency. In these cases, the RF signal received is difficult to process. Therefore, in such situations, the RF communication device must be able to amplify or provide gain to the RF signal received which originates from a far away source.
For example, when an operator is using a RF communication device remotely located from the RF transmitting source, the RF amplifier is engaged to amplify the incoming RF signal (an electromagnetic signal or wave) to allow processing by processing circuitry. When the operator is closer to the RF transmitting source, the incoming RF signal is relatively strong (corresponding to a high electromagnetic signal) which means that the RF amplifier must switch to a lower gain state (or amplification state). If the large RF signal continues to be amplified when near a RF transmitting source, the RF communication device""s processing circuitry would be overloaded and will not function. Thus, a lower gain amplification state is required when near a RF transmitting source.
Linearity is an important characteristic in the design of wireless communications devices because linear amplifiers provide an RF amplified signal without, for example, degradation of the signal due to third order intermodulation. Usually, RF wireless devices employ bipolar transistor technology to amplify the incoming RF input signal. While most prior art RF variable gain bipolar amplifiers can change the amount of amplification or gain applied to the received RF signal by changing the direct current (DC) bias current flowing through the amplifiers, such amplifiers suffer because the amplifiers employed are not linear in the low gain mode because of the low DC bias current. Thus, if the RF signal contains additional unwanted large amplitude signals (such as, for example, interferers, which differs slightly from the desired RF signal in frequency), and these signals experience third order nonlinearity from the amplification process of the amplifier, the resulting effect of the third order nonlinearity between the two interferers can be a false signal created on the same frequency as the desired amplified signal. This resulting signal is known as the third order intermodulation product. The third order intermodulation product is a signal at the same frequency of the desired RF signal which corrupts the desired RF signal. In any event, the resulting intermodulation product signal may be difficult to process correctly by the RF communication device""s processing circuitry.
Maintaining linearity of the RF amplifier is therefore an important consideration in RF communication design because a highly linear RF amplifier will be able to provide the desired RF signal to the RF communication device""s processing circuitry at the correct frequency while minimizing the effects of third order intermodulation. Thus, there is a need for a method for variable linear amplification and an apparatus for a variable linear amplifier, particularly suited for RF communication applications, which amplifies in a linear fashion so that a desired RF input signal corresponds to a desired RF output signal transmitted to the RF communication device""s processing circuitry without corruption from third order intermodulation. Further, it is desirable that the amplifier in RF communication devices should be able to maintain a constant input and output impedance from the low gain to high gain mode so that the filters before and after the amplifier will operate properly.